Separation of hydrocarbons



United States Patent '9 2,799,629. v SEPARATION or HYDROCARBONS Harry Clough and John Allan Smith, Norton-on-Tees, England, assignors to Imperial Chemical Industries Limited, a corporation of Great Britain No Drawing. Application November 24, 1952, Serial No. 322,344

Claims. priority, application Great Britain December 14, 1951 3 Claims. (Cl. 202-395) This invention relates to the separation of hydrocarbons.

It is known broadly to separate aromatic hydrocarbons from non-aromatic hydrocarbons by extractive distillation using various solvents such as anilines and phenols.

It is commonly desired to separate fractions having an increased xylene and areduced ethyl benzene content from complex mixtures of xylenes with ethyl benzenes and aliphatic and/or naphthenic hydrocarbons, but this has been found to be difficult. The boiling points at atmospheric pressure of ethyl benzene, o-xylene, m-xylene and p-xylene, lie within a relatively narrow temperature range. The application of fractional distillation, azeotropic distillation or extractive distillation to the aforesaid complex mixtures has not previously been found to give altogether satisfactory results from the aspects of efficiency and economy, and this no doubt is largely attributable .to the proximity of the boiling points and the similarity in chemical properties, of the components.

It is important to be able to separate mand p-xylenes from ethyl benzene in order to provide a suitable feedstock for isomerisation of the xylenes, a. process of growing importance for the production of p-xylene. The presence of ethyl benzene in such feedstock results in degradation, reduces the total output of the isomerisation unit, leads to the production of impure xylenes, and, if it is desired to separate the xylenes inter se, interferes with that separation. Moreover, separation of the ethyl benzene is additionally advantageous since it is a valuable intermediate for the production of styrene. The present invention provides an efiicient process for ob taining an aromatic fraction enriched in xylenes and impoverished in ethyl benzene from hydrocarbons containing non-aromatics, ethyl benzene and xylenes.

According to the present invention there is provided a process for the production of a fraction having an increased xylene content and an increased weight ratio xyleneszethyl benzene, and of a fraction having an increased weight ratio ethyl benzenez xylene's, from mixtures comprising ethyl benzene, xylenes and aliphaticand/or naphthenic saturated hydrocarbons which comprises subjecting the mixture to extractive distillation with a polar solvent having a boiling point at least 50 C. higher than that of o-xylene, taking off as overhead prod not a fraction containing substantially all the non aromatic hydrocarbons and only a minor proportion of xylenes, and as bottom product a solution of aromatic hydrocarbons substantially free from nonraromaticsand having an increased xylene content and. an increased weight ratio, xyleneszethyl benzene.

It is preferred that the percentage by weight of xylenes in the overhead fraction from the extractive column be kept as low as possible, and in any case it should not exceed' 10%, and preferably should not exceed 5%.

Examples of suitable polar solvents are alkyl phenols,

ice

umn, the feed at a point below this, a mixture of nonaromatic hydrocarbons and ethyl benzene containing a small amount of xylenes is taken off as overheads, and a solution of xylenes in solvent as bottoms. This last mentioned solution is delivered to the stripping column, in which mixed xylenes are taken olf as overhead, and the solvent is obtained as bottoms and returned to the extractive distillation column. There is a reflux of overheads to the extractive column to scrub out solvent from the vapours, and a reflux of bottoms to the extractive column to scrub out non-aromatics from the liquid product. In order to increase the concentration of xylenes in the bottom product it is desirable to permit a minor proportion of xylene to remain in the overhead.

It is convenient to operate both columns at atmospheric pressure, but reduced or elevated pressure may also be used.

An advantage connected with the use as solvent of the aforesaid phenol cut boiling within the range 2l5- 255 C. is that it makes possible animproved separation of pand m-xylenes from ethyl benzene, which is surprising since the boiling points at atmospheric pressure of the three compounds are very close, being 138.5, 138.8 and 136.2 C., respectively. Other advantages attending the use of this solvent are: it does not azeotrope to a serious degree with any of the components of the aforesaid type of hydrocarbon mixture; it is non-corrosive; the toxic hazard with it is low; it is stable; its cost is low; and it can be easily recovered in a sufficiently pure condition for recycling.

The following table illustrates the low azeotroping power of this solvent as compared with other solvents that have been examined. The method of test was to fractionate synthetic mixtures of solvent (i) with nonarornatics and (ii) with aromatics and determine the composition of the overhead product:

Solvent (1) Azeotrope with nou- (1i) Azeotrope with aromatle hydrocarbons aromatic hydrocarbons Phenol Azeotrope containing Azeotrope containabont 8.5% phenol. ing about 5.5%

eno metacresylic acid Azeotrope containing Az eotrope contain- 0 about 3% cresols. ing 0.7% cresols. 230-950 0. cut of tar Azeotrope containing No azeotrope is acids. about 1% tar acids. 1 formed.

over methods employing, azeotropic distillation .are that for 'a given throughput: fewer stills and" less. equipment are needed; the heat consumption is considerably less;

the product is purer; the solvent is cheaper; and operation is simpler and easier.

The concentration of non-aromatic hydrocarbons is determined chromatographically, and of xylenes by infra red spectroscopy.

EXAMPLE 1 The mixture of hydrocarbons to be treated had the following composition.

Percent by weight p-Xylene 1 7.0 m-Xylene 1 16.4 .o-Xylene 1 2.9 Ethyl benzene 7.0 Non-aromatics 66.7

1 Xylenes ethyl benzene: 3.8 1.

C. Initial boiling point 218.5 95% had distilled at 245.5

was fed at a rate of 283 grams per hour to the extractive distillation column at a point 7 /2" below the top of the column, and the aforesaid hydrocarbon mixture at a point 31 /2 below the top of the column at a rate of 67.9 grams per hour.

A mixture having the composition:

' Percent by weight Non-aromatics 89.00 Ethyl benzene 5.54 Xylenes (total) 5.46 p-Xylene 2.42 m-Xylene 3.04

was taken oil as overheads at the rate of 50.8 grams per hour; and a phenol solution of xylene concentrate together with a small proportion of non-aromatics and a reduced content of ethyl benzene having the relative composition:

Percent by weight o-Xylene 14.0 m-Xylene 1 55.8 p-Xylene 1 20.0 Ethyl benzene 10.0 Non-aromatics 0.2

1 Xylenes ethyl benzene=9 1.

was taken off as bottoms. The column was run under the following equilibrium conditions:

C. Still head temperature 130 Phenols inlet temperature 148 Hydrocarbon inlet temperature 131 Boiler temperature 208 Approximately 84.5% by weight of the xylenes present in the original mixture was recovered in the bottom product.

These data show that operating as described it is possible to obtain overhead a fraction enriched in ethyl benzene, and as bottoms a solution of aromatics in phenol substantially free from'non-aromatics and showing an increased content of .xylenes, and a lower ethyl benzene: xylenes ratio. It is a simple matter to treat the bottoms in a stripping column and obtain as overhead a xylene mixture having only small amounts of non-aromatics and a lower ethyl benzene2xylenes ratio than the starting material. The use of the aforesaid phenol cut as solvent in the extractive distillation facilitates the separation.

In this extractive distillation, the ratio of ethyl benzene to p-xylene has been raised from 50:50 in the feed to 70:30 in the distillate. The normal relative volatility of ethylbenzene with respect to p-xylene is 1.07 and the above degree of separation would have required a reflux ratio of over 10:1 even with an infinite number of plates. In the presence of the solvent and the non-aromatics, however, the separation was in fact achieved in a length of column corresponding to 24 theoretical plates at a reflux ratio of less than 5. This means that the effective relative volatility of ethyl benzene to p-xylene must have been increased to at least 1.25.

A similar enhancement of relative volatility was obtained in the extractive distillation of the same initial mixture using the same phenol cut in a 56 inches long by 1.25 inches internal diameter column packed with /8 inch gauze rings. Here operation at a 2:1 reflux ratio gave an overhead product having an aromatic hydrocarbon content or" which 70% by weight was ethyl benzene, a concentration which, in the absence of the added polar solvent, could have been achieved only by the use of reflux ratios greater than 10:1 and of a very much longer column. The benefits in heat economy are evident.

EXAMPLE 2 A similar extractive distillation was carried out on a hydrocarbon mixture having the following composition:

' Percent by weight p-Xylene 1 4.3

m-Xylene 1 8.6 o-Xylene 6.9 Ethyl benzene 4.1 Non-aromatics 76.1

1 Xylenes ethyl benzene: 4.8: 1.

The extractive distillation column was 56 inches long by 1.25 inches internal diameter, was packed with 44" stainless steel gauge ferrules and in hydrocarbon fractionation had an efliciency of 36 theoretical plates. The characteristics of the extractive solvent were:

Initial boiling point, 221 C. had distilled at 250 C.

Overhead fraction Percent by weight Non-aromatics 91.1 Ethyl benzene 3.2 Xylenes (total) 5.7 p-Xylene 2.0 m-Xylene 3.4 o-Xylene 0.3

Bottom fraction Percent by weight o-Xylene 1 33.8 m-Xylene 1 36.7 p-Xylene- 15.5 Ethyl benzene 6.7 Non-aromatics 7.3

1 Xylenes ethyl benz ene=12.8 1.

Approximately 78% by weight of the xylenes present in the original mixture was recovered in the bottom product. The lower recovery of xylenes is attributable to the lcanness in xylenes of the starting material.

EXAMPLE 3 A similar extractive distillation was carried out employing the same column and extractive solvent as in Example 2, on a hydrocarbon mixture having the composition:

Percent by weight p-Xylene 1 6.6 m-Xylene 1 12.0 o-Xylene 12.0 Ethyl benzene 10.0 Non-aromatics 59.4

1 Xylenes ethyl benzene: 3.1 1.

The compositions of the hydrocarbons taken off from the extractive column as overheads and, as bottoms, in

1 Xylenes ethyl benzene 5.5 l.

The column was operated so that 61.7% by weight of the feed was taken overhead, the still head temperature being 134, the boiler temperature 194, and the feed inlet and solvent inlet temperatures 118 C. and 150 C. respectively.

Approximately 85% of the xylenes present in the original mixture was recovered in the bottom product.

The invention is of particular value in its application to complex hydrocarbon mixtures of the specified type boiling between 125 and 155 C.

Preferably, because more effective separation results, the starting material should contain at least 40%, and more preferably at least 60%, by weight of non-aromatic hydrocarbons. The invention is applicable with considerable advantage to such mixtures in which the proportion of ethyl benzene amounts to from 5 to 30% by weight of the total xylenes, since thereby more valuable ethyl benzene and xylene concentrates are obtained. Preferably, for similar reasons, the xylene content of the starting material should not be less than 5% by weight, and more preferably not less than 20% by weight thereof.

The invention may also be applied to hydrocarbon mixtures which are deficient in saturated aliphatic and/or naphthenic hydrocarbons by making up the deficiency by the addition of saturated aliphatic and/or naphthenic hydrocarbons before applying the extractive distillation.

We claim:

1. Process for the production of a fraction with an increased xylene content and an increased weight ratio of xylenezethyl benzene and a fraction with an increased ethyl benzenecxylene Weight ratio from feed mixtures containing ethyl benzene, xylenes and at least one nonaromatic hydrocarbon selected from the group of saturated hydrocarbons consisting of aliphatic and naphthenic hydrocarbons, characterized in that said feed mixture contains at least 40% of non-aromatic hydrocarbons, which comprises subjecting said feed mixture to extractive distillation with an excess of a substituted mono-hydroxy phenol cut having a boiling point within the range of from 215 to 255 C. at atmospheric pressure and containing not more than 10% by weight of phenols in which the 2- and 6-positions, relative to the hydroxy group, are occupied by alkyl radicals, and taking 011 as overhead product a fraction containing substantially all of said non-aromatic hydrocarbons, a major proportion of said ethyl benzene and only a minor proportion of xylenes, and as bottom product a solution of aromatic hydrocarbons substantially free from said non-aromatic hydrocarbons and comprising the major proportion of said xylenes and only a minor proportion of said ethyl benzene, so that said bottom product has a substantially increased weight ratio of xyleneszethyl benzene relative to the ratio in said feed mixture.

2. A process as claimed in claim 1 in which the nonaromatic content by weight of the starting material is less than 40% and prior to the extractive distillation there is introduced there into suflicient non-aromatic hydrocarbon to raise the non-aromatic content by weight to at least 40% by weight of the resulting mixture.

3. The process of claim 1 wherein said minor proportion of xylene in said overhead product is maintained below about 5%.

References Cited in the file of this patent UNITED STATES PATENTS 2,286,503 Ocon June 16, 1942 2,288,126 Dunn et a1 June 30, 1942 2,357,028 Shiras et al Aug. 29, 1944 2,366,570 Souders et a1 Jan. 2, 1945 2,370,530 Gage Feb. 27, 1945 2,375,035 Pierotti May 1, 1945 2,496,253 Purcell Jan. 31, 1950 2,721,170 Johnson Oct. 18, 1955 2,763,604 Dorsey et al Sept. 18, 1956 

1. PROCESS FOR THE PRODUCTION OF A FRACTION WITH AN INCREASED XYLENE CONTENT AND AN INCREASED WEIGHT RATIO OF XYLENE:ETHYL BENZENE AND A FRACTION WITH AN INCREASED ETHYL BENZENE:XYLENE WEIGHT RATIO FROM FEED MIXTURES CONTAINING ETHYL BENZENE, XYLENES AND AT LEAST ONE NONAROMATIC HYDROCARBON SELECTED FROM THE GROUP OF SATURATED HYDROCARBONS CONSISTING OF ALIPHATIC AND NAPHTHENIC HYDROCARBONS, CHARACTERIZED IN THAT SAID FEED MIXTURE CONTAINS AT LEAST 40% OF NON-AROMATIC HYDROCARBONS, WHICH COMPRISES SUBJECTING SAID FEED MIXTURE TO EXTRACTIVE DISTILLATION WITH AN EXCESS OF A SUBSTITUTED MONO-HYDROXY PHENOL CUT HAVING A BOILING POINT WITHIN THE RANGE OF FROM 215* TO 255*C. AT ATMOSPHERIC PRESSURE AND CONTAINING NOT MORE THAN 10% BY WEIGHT OF PHENOLS IN WHICH THE 2- AND 6-POSITIONS, RELATIVE TO THE HYDROXY GROUP, ARE OCCUPIED BY ALKYL RADICALS, AND TAKING OFF AS OVERHEAD PRODUCT A FRACTION CONTAINING SUBSTANTIALLY ALL OF SAID NON-AROMATIC HYDROCARBONS, A MAJOR PROPORTION OF SAID EHTYL BENZENE AND ONLY A MINOR PROPORTION OF XYLENES, AND AS BOTTOM PRODUCT A SOLUTION OF AROMATIC HYDROCARBONS SUBSTANTIALLY FREE FROM SAID NON-AROMATIC HYDROCARBONS AND COMPRISING THE MAJOR JPROPORTION OF SAID XYLENES AND ONLY A MINOR PROPORTION OF SAID ETHYL BENZENE, SO THAT SAID BOTTOM PRODUCT HAS A SUBSTANTIALLY TO THE RATIO IN SAID FEED MIXTURE. 